The chuck on electric power drills is customarily mounted by threading the chuck onto the end of the drive spindle until the chuck body seats against a reference surface on the spindle. This squares the chuck relative to the spindle while the threads center the chuck on the spindle. These threads in the chuck body are utilized in manufacture of the chuck for properly positioning the chuck body for final grinding of the jaws so the jaws will be concentric with the spindle.
In use, power is delivered from the motor to the chuck through the threaded engagement. Threads operate as a wedge. The greater the resistance encountered by the chuck the more the threads tend to tighten. While this is true with a regular chuck, it is particularly true with an impacting chuck since such chucks are subjected to force spikes during the impact process. Whatever type of chuck one is discussing, it can be difficult to remove the chuck for service simply because the chuck is thoroughly jammed on the drive spindle and some rather extreme measures may be necessary in order to free the chuck from the spindle. This makes service difficult, expensive and can result in permanent damage to the chuck or to the drill.
Keyless chucks rely on impact to open and close the chuck. The design is such that the impacting is more severe in the reverse direction and the chucks have been known to loosen from the spindle thread resulting in applying that wedging effect on the retaining screw causing it to break.